Optimised Positive Electrode Material for Lithium Cell Batteries, Method for the Production Thereof, Electrode, and Battery for Implementing Said Method

ABSTRACT

A material or compound is provided having a spinel structure and corresponding to the formula Li y Ni 0.5 Mn 1.5-x   IV Mn x   III A z O 4-d , where:
         0.02≦x≦0.35;   d&gt;0;   A is selected from the group comprising Na, K, Mg, Nb, Al, Ni, Co, Zr, Cr, Fe, Cu, Ti, Zn, Si and Mo;   0.8≦y≦1.2;   0≦z≦0.1; and has a mesh parameter of between 8.174 and 8.179 Å.

TECHNICAL FIELD

The present invention relates to a novel compound with a spinelstructure, of the mixed oxide type based on Ni, Mn and Li,non-stoichiometric and having a clearly defined mesh parameter.

Such a compound has optimised properties, in terms of stability andelectrochemical performance.

In consequence, it is advantageously used in electrodes, batteries andcell batteries containing lithium.

PRIOR ART

Lithium cell batteries are increasingly used as self-contained energysources, particularly for portable equipment, where they progressivelytend to replace nickel-cadmium (NiCd) and nickel-metal hydride (NiMH)batteries. This development stems from the fact that the performance oflithium cell batteries, in terms of power density (Wh/kg, Wh/l), issubstantially superior to that of the two abovementioned technologies.

The active electrode compounds used in these batteries mainly have theformulas LiCoO₂, LiNiO₂ and LiMn₂O₄ for the positive electrode, andcarbon (graphite, coke, etc.) for the negative electrode. Thetheoretical and practical capacitances of these compounds arerespectively 275 mAh/g and 140 mAh/g for LiCoO₂ and LiNiO₂, and 148mAh/g and 120 mAh/g for LiMn₂O₄, for an operating voltage close to 4volts with respect to lithium metal.

The existing compound LiNi^(II) _(0.5)Mn^(IV) _(1.5)O₄, having a spinelstructure, is electrochemically active reversibly at a potential of4.7-4.8 V/Li/Li⁺. Its theoretical specific capacitance is 147 mAh/g.This gives it a theoretical power density of about 700 Wh/Kg/Li⁺/Li.

Despite numerous and varied studies on materials derived from LiMn₂O₄,few data are available on the compound LiNi_(0.5)Mn_(1.5)O₄. The maindata available concern the optimisation of its synthesis and theelectrochemical behaviour and performance of this material, as positiveelectrode for a lithium battery.

It should be noted that the term “electrochemical performance” of amaterial pertains to two different concepts:

the capacity to respond to high current (power) densities; and

the capacity to have a long cycling service life (high and stablecapacitance in cycling).

These two properties can be obtained simultaneously for a givencompound, but this simultaneous feature is not systematically obtained.Depending on the target application, one and/or the other of theseproperties is therefore desirable.

Document JP-A-8 217452 discloses a material with a spinel structure,with the formula LiNi_(x)Mn_(2-x)O₄, in which x is between 0 and 0.5inclusive. It is reported that this compound has good cycling properties(without specification), due to the use, as precursor, of a manganeseoxide MnO₂ having a specific surface area of 150 to 500 m²/g.

Document FR 2 831 993 relates to the doping of a compound with thegeneral formula LiMn_(2(x+y))M_(x)M′_(y)O₄ by element M′, with M═Ni orCo and M′═Ti, Al, Co, Mo, x and y strictly positive. This results in amaterial which operates well at high potential, with high capacitanceand good cycling properties.

Furthermore, other studies, such as the publication of Sun et al.(Electrochimica Acta 48(2003) 503), recommend a surface treatment of thematerial (based on ZnO for example) to improve the cycling performance.

Even more recently, Kim et al. (Chem. Mat. 16(2004), 906) showed that aderivative of LiNi_(0.5)Mn_(1.5)O₄, non-stoichiometric, with a spinelstructure Fd-3m, and a formula Li_(1.00)Ni_(0.48)Mn_(1.5)O_(3.92) andmesh parameter 8.172 Å, compared with its purely stoichiometrichomologue with a mesh parameter of 8.166 Å, has a better insertionkinetics, and hence better response at high current density.

Thus, values of 137 mAh/g have been obtained for the two compounds atC/7, while at 3C, a value of 110 mAh/g was measured for thenon-stoichiometric compound, against 60 mAh/g for the stoichiometriccompound. These better properties have been attributed by the authors tothe difference in stoichiometry alone, although other crucial factors,particularly the morphology and grain size, have not been taken intoaccount.

Patent application JP 2002 158007 A also relates to the compound withthe formula LiNi_(0.5)Mn_(1.5)O₄, with mesh parameters of 8.18 Å ormore. It is certainly demonstrated that the mesh parameters of between8.174 and 8.179 Å can be obtained, but only after heat treatment, andhence a modification of the composition. Furthermore, theMn^(III)/Mn^(IV) distribution in this compound remains undetermined.

Methods for synthesising this class of compounds have also beendescribed in (Perentzis et al., J. SOLID STATE ELECTROCHEM. 8(1) 2003,51-54; Ito et al., JOURNAL OF POWER SOURCES 119-121 (2003), 733-37).

The present development of lithium cell batteries reveals the need toidentify novel compounds, serving as a positive electrode material andhaving optimised electrochemical properties, particularly in terms ofcycled capacitance and the cycling stability of this capacitance.

SUMMARY OF THE INVENTION

In the context of the invention, the Applicant has obtained compoundswith a spinel structure, of the mixed oxide type based on nickel,manganese and lithium, non-stoichiometric, with novel and clearlydefined formulas and morphology, having the desired properties.

Thus, the present invention relates to a compound having a spinelstructure, with the formula Li_(y)Ni_(0.5)Mn_(1.5-x) ^(IV)Mn_(x)^(III)A_(z)O_(4-d), with:

0.02≦x≦0.35;

d>0;

A selected from the group comprising Na, K, Mg, Nb, Al, Ni, Co, Zr, Cr,Fe, Cu, Ti, Zn, Si and Mo;

0.8≦y≦1.2;

and having a mesh parameter of between 8.174 and 8.179 Å.

In this formulation, x hence quantifies the manganese content atoxidation degree III, d an oxygen deficiency and z the contribution ofdoping element A.

In general, a compound of the invention has the formulaLi_(y)Ni_(0.5)Mn_(1.5-x) ^(IV)Mn_(x) ^(III)O_(4-d).

Advantageously, the formula of such a compound is LiNi_(0.5)Mn_(1.3)^(IV)Mn_(0.2) ^(III)O_(3.9).

According to another embodiment, the compound has a formula in which zis higher than 0, that is 0<z≦0.1, indicating the presence of a dopingelement A.

The compound of the invention proves to have optimised properties interms of stability and electrochemical performance in the context of itsuse in a lithium cell battery, during successive cyclings. Thus, valuesof 120 to 140 mAh/g have been observed at C/5, with 0.03 to 0.30% ofloss per cycle over a hundred cycles.

In fact, the Applicant has found that in general, a stoichiometriccompound had a high cycling loss, about 9% per cycle, under the testconditions, while all other things remaining equal (particularly atequivalent specific surface area), derivative non-stoichiometriccompounds displayed improved capacitance preservation. With a view toindustrial application, the rates of loss per cycle required mustgenerally be about 0.04-0.05%. For the inventive compounds, it waspossible to identify a window of composition in which the experimentalvalues obtained converge towards such values. This window corresponds tospinel parameters of between 8.174 and 8.179 Å, with an optimum for8.176-8.177 Å.

Advantageously, the inventive compounds therefore have a mesh parameterof between 8.176 and 8.177 Å.

The abovementioned document Kim et al certainly and undeniably revealedthe advantage of a non-stoichiometric compound, but for a different meshparameter and for kinetic properties, but on the contrary, did notaddress the problem of cycling stability. In fact, as already stated,these properties are basically decorrelated, indeed antithetical. Thisis because the cycling stability first demands structural stability andcan then be improved by decreasing the area in contact with theelectrolyte (larger particle size, lower specific surface area). On thecontrary, improved kinetics generally demands the reduction of thediffusion paths, conventionally obtained by decreasing the grain size,and therefore implying an increase in specific surface area.

It is moreover desirable for the inventive compound to have a clearlydefined morphology, that is, to have a particle size higher than 1 μm,preferably of between 5 and 10 μm, and specific surface area of between1 and 2 m²/g.

This particular morphology serves to limit the reactivity to theelectrolyte at high potential and therefore also contributes to limitthe capacitance losses during cycling, at ambient temperature and at 55°C., the temperature at which reactivity is usually amplified.

A perfect control of the synthesis parameters, particularly with thetemperature and treatment time and the type of cooling employed, arenecessary to obtain the inventive compounds. The invention thereforealso relates to the method for preparing the said compounds. Althoughseveral combinations are conceivable, the essential steps of such amethod are as follows:

mixing of the precursors in stoichiometric conditions with a lithiumexcess of 1 to 5% mole;

the mixture is subjected to a first heat treatment at a temperature ofbetween 550 and 600° C.;

the mixture is subjected to a second heat treatment at a temperatureabove 700° C., preferably above 800° C., combined with a cooling inoxygen containing medium, to obtain the anticipated mesh parameter.

During the first step, the precursors required to obtain the desiredcomposition are therefore intimately mixed in stoichiometric conditions,with a lithium excess of 1 to 5 mol %. The precursors are typicallybased on carbonates (nickel carbonate, lithium carbonate and manganesecarbonate) or on beta manganese oxide, lithine (LiOH.H₂O) and nickeloxide (NiO).

The first heat treatment at high temperature (about between 550 and 600°C.) lasting several hours must permit the proper incorporation of thelithium with the mixed Ni and Mn oxide.

The second heat treatment, at even higher temperature (above 700° C. andpreferably above 800° C.), lasting several hours, is intended to cause aloss of oxygen, while creating the desired morphology. Advantageously,this treatment is commensurately longer as the treatment temperature islower.

Finally, a cooling (more or less slow), adapted to the precedingtreatment, is carried out in oxygen containing medium, to permit apartial oxygen return. A certain quantity of manganese with oxidationdegree III must persist, corresponding to the range of mesh parametersof the invention, on completion of the protocol. A compromise betweenthe second treatment and the type of cooling serves to obtain theinventive product.

Due to the advantageous electrochemical properties of the claimedcompound, the invention also relates to the electrodes, the cellbatteries which have at the negative electrode either lithium metal(lithium metal type), or a lithium insertion material (lithium-ion type)comprising the said compound as the active material, and moreelaborately, batteries consisting of cells in series.

The electrodes are preferably composed of a nanodispersion of activematerial, with an electron conducting additive (for example carbon)and/or an organic binder conferring ion conduction and mechanicalproperties (for example polyether, polyester, polymers based onmethylmethacrylate, acrylonitrile, vinylidene fluoride), deposited onmetal sheets serving as current collectors. A mechanical separatorbetween the two electrodes is soaked with electrolyte (ion conductor).This generally consists of a salt, whereof the cation is at least partlythe lithium ion (LiClO₄, LiAsF6, LiPF₆, LiBF₄, LiCH₃SO₃, etc.), and anaprotic polar solvent (ethylene or propylene carbonate,dimethylcarbonate, diethylcarbonate, methylethylcarbonate, etc.).

All of these devices confer the requisite performance described above.

EXEMPLARY EMBODIMENT

The invention and the advantages obtained emerge clearly from the saidexemplary embodiments which follow. However, these examples are not atall limiting.

Example 1

An intimate mixture of precursors based on carbonates (10.535 g nickelcarbonate, 6.468 g lithium carbonate and 30.090 g manganese carbonate)in stoichiometric conditions with an Li excess of 3 mol % is prepared bygrinding with a Retsch type planetary grinder in a 250 ml bowlcontaining ten balls 20 mm in diameter, for 20 hours at 400 rpm, in thepresence of hexane in sufficient volume to submerge the powder. Themixture is dried overnight at 60° C., and then treated at 600° C. (10hours), and then at 900° C. (15 hours), followed by cooling to ambienttemperature at the rate of 1° C./min. The compound obtained has theproperties of the invention.

Example 2

In another embodiment, the precursors used consist of beta typemanganese dioxide, lithine (LiOH.H₂O) and Ni oxide, NiO. An intimatemixture of the precursors is prepared by grinding with a Retsch typeplanetary grinder for two hours in the presence of hexane. The mixtureis then dried overnight at 60° C. and then treated at 550° C. for 60hours, then at 700° C. (15 hours), followed by return to ambienttemperature at the rate of 20° C./min. The compound obtained also hasthe properties of the invention.

Example 3

A lithium metal battery is prepared comprising a negative electrodeconsisting of a disc of lithium (diameter 16 mm, thickness 130 microns)deposited on a nickel disc serving as a current collector, a positiveelectrode consisting of a 14 mm disc taken from a 50 micron thickcomposite film comprising the inventive material prepared according toexample 1 (8% by weight), carbon black (8% by weight) as conductingmaterial and polyvinylidene hexafluoride (12% by weight) as binder, thewhole being deposited on an aluminium current collector (25 micron thicksheet) and a separator soaked with liquid electrolyte 1M LiPF₆ insolution in propylene carbonate. At 25° C., such a battery delivers acapacitance of 130 mAh/g, stable in cycling at C/5 (0.1% losses percycle).

Example 4

A lithium ion battery is prepared with a composite electrode containingthe active material Li₄Ti₅O₁₂ (negative electrode), a compositeelectrode containing the inventive material prepared according toexample 1 (positive electrode), a separator soaked with liquidelectrolyte consisting of 1M LiPF₆ in solution in propylene carbonate.The composite electrode Li₄Ti₅O₁₂ consists of 80% by weight of activematerial, carbon black (8% by weight) as conducting material, andpolyvinylidene hexafluoride (12% by weight) as binder, the whole beingdeposited on an aluminium current collector. The performancecharacteristics are similar to those obtained in example 3.

Example 5

A prototype Li-ion cell battery (prototype wound with double-sidedelectrodes) of 450 mAh, consisting of the inventive material coupledwith Li₄Ti₅O₁₂ was prepared in a flexible packing from thick titania andspinel electrodes coated with 300 μm (1.5 mAh/cm²/side). Theseprototypes of 30 Ah/kg have similar capacitances in operation to thoseobtained at smaller scale.

1. Compound having a spinel structure, wherein: the compound has theformula Li_(y)Ni_(0.5)Mn_(1.5-x) ^(IV)Mn_(x) ^(III)A_(z)O_(4-d), with:0.02≦x≦0.35; d>0; A selected from the group comprising Na, K, Mg, Nb,Al, Ni, Co, Zr, Cr, Fe, Cu, Ti, Zn, Si and Mo; 0.8≦y≦1.2; 0≦z≦0.1; andhas a mesh parameter of between 8.174 and 8.179 Å.
 2. Compound accordingto claim 1, having a formula Li_(y)Ni_(0.5)Mn_(1.5-x) ^(IV)Mn_(x)^(III)O_(4-d).
 3. Compound according to claim 2, having a formulaLiNi_(0.5)Mn_(1.3) ^(IV)Mn_(0.2) ^(III)O_(3.9).
 4. Compound according toclaim 1, wherein z is higher than
 0. 5. Compound according to claim 1having a mesh parameter of between 8.176 and 8.177 Å.
 6. Compoundaccording to claim 1 having a particle size above 1 μm.
 7. Compoundaccording to claim 6 having a particle size of between 5 and 10 μm. 8.Compound according to claim 1 having a specific surface area of between1 and 2 m²/g.
 9. Method for preparing a compound according to claim 1,comprising the following steps: forming a mixture by mixing of lithium,nickel and manganese based precursor in stoichiometric conditions with alithium excess of 1 to 5 mol %; subjecting the mixture to a first heattreatment at a first temperature of between 550 and 600° C.; after thefirst heat treatment, subjecting the mixture to a second heat treatmentat a second temperature above 700° C. combined with a cooling in oxygencontaining medium.
 10. Electrode comprising a compound according toclaim 1, as an electrochemically active material.
 11. Electrodeaccording to claim 10, wherein the compound is in the form of adispersion mixed with a conducting additive and/or an organic binder.12. Cell battery comprising at least one first electrode according toclaim 10, and a second electrode made from a material suitable forreceiving lithium ions, first electrode and the second electrode beingseparated by a separator soaked with electrolyte.
 13. Battery comprisingat least one cell battery according to claim
 12. 14. Method according toclaim 9, wherein the second temperature is above 800° C.